Acrylic resins as typified by PMMA have an excellent optical performance and are therefore applied to various optical materials used as optical isotropic materials having high light transmittance, low refractive index, and low retardation. However, along with the latest developments in flat displays such as liquid crystal display devices, plasma displays, and organic EL display devices, infrared sensors, optical waveguides, and the like, there are increased demands for optical transparent polymer materials and particularly planar (e.g., film- or sheet-formed) optical transparent polymer materials.
As the characteristics required for planar optical transparent polymer materials, high transparency and high optical anisotropy are mentioned, and characteristics, for example, heat resistance, corresponding to applications, are required together with the above characteristics.
It is demanded of optical protective films used in polarizing plates and the like to have, besides high transparency and high optical isotropy, characteristics such as low photoelastic coefficient, high heat resistance, high light resistance, and high surface hardness, high mechanical strength, small wavelength dependency of the retardation, and small incident angle dependency of the retardation.
It is required for optical films such as retardation films and view angle compensation films to have, besides high transparency and high optical isotropy, characteristics such as low photoelastic coefficient, high heat resistance, high light resistance, high surface hardness, high mechanical strength, high retardation, small wavelength dependency of the retardation, and small incident angle dependency of the retardation.
It is required for optical sheets such as diffusing plates and light guide plates to have, besides high transparency and high optical isotropy, characteristics such as low photoelastic coefficient, high heat resistance, high light resistance, high surface hardness, and high mechanical strength. It is particularly desired to suppress a deterioration in optical characteristics caused by formation strain by providing higher heat resistance than polymethylmethacrylate (PMMA) used in current optical sheets.
However, conventional planar optical transparent polymer materials insufficiently satisfy the requirements for these characteristics.
On the other hand, lactone ring-containing polymers obtained by allowing a polymer having hydroxyl groups and ester groups in its molecular chain to cause lactone cyclization condensation reaction are known (see, e.g., Japanese Unexamined Patent Publications Nos. 2000-230016, 2001-151814, 2002-120326, and 2002-254544). However, because these polymers can insufficiently exhibit optical isotropy, light resistance, and surface hardness and can satisfy the requirements of mechanical strength, there has been no case in which these polymers are made into films or sheets to form planar optical transparent polymer materials. Further, in the case of increasing the content of a lactone ring structure to improve heat resistance, the optical isotropy of a film tends to deteriorate and there is therefore the case where it is difficult to obtain an optical film having low birefringence and low retardation when the lactone ring-containing polymer is only used.